Dissolved ammonia catalyzes proto-dolomite precipitation at Earth surface temperature

Abstract

Marine carbonate rocks are the major reservoir of carbon through CaCO₃ and CaMg(CO₃)₂ precipitation extracting CO₂/HCO₃^- from the atmosphere/oceans; hence dolomite is one of the major sinks in the global carbon cycle. Most ancient dolomite has been considered as mainly precipitated under Earth surface conditions. Previous studies have demonstrated that microbes can mediate dolomite formation. However, the “microbial dolomite” model is not sufficient to explain the dolomite that shows no or little evidence of a microbial origin. Although attempted for decades, the synthesis of inorganically-produced dolomite at normal temperatures (<50 °C) has been relatively unsuccessful. Hence, this "dolomite enigma" has been one principal research focus this century. Here we demonstrate that NH₃ catalyzes proto-dolomite precipitation inorganically with higher Mg/Ca molar ratios and CO₃^2- activity at normal temperatures of 30 °C and 40 °C. NH₃ dissolution increases the alkalinity of the solution and transformes into NH₄⁺ ions, which prefer to bond with H₂O on Mg[(H₂O)₆]²⁺ rather than free H₂O, thus releasing Mg²⁺ to facilitate proto-dolomite nucleation. Furthermore, the low dielectric constant and low dipole moment allow NH₄⁺ absorbed on crystal surfaces to lower the energy barrier of Mg[(H₂O)₆]²⁺ dehydration, promoting proto-dolomite nucleis growth. The system for proto-dolomite precipitation in our experiments closely simulates the natural aqueous environment. This study brings new insights to understanding the mechanisms of dolomite precipitation in natural waters.